1. Field of the Invention
The present invention is related to medical devices and more particularly to ultrasound imaging.
2. Background
Ultrasound imaging is a common method of analysis used for examining a wide range of materials. The method is especially common in medicine because of its relatively non-invasive nature, low cost, and fast response times. Typically, ultrasound imaging is accomplished by generating and directing ultrasonic sound waves into a material under investigation in a transmit phase and observing reflections generated at the boundaries of dissimilar materials in a receive phase. For example, reflections are generated at boundaries between a patient's tissues. The reflections are converted to electrical signals by receiving devices (transducers) and processed, using beam-forming techniques known in the art, to determine the locations of echo sources. The resulting data is displayed using a display device such as a monitor.
Typically, the ultrasonic signal transmitted into the material under investigation is generated by applying continuous or pulsed electronic signals to a transducer. The transmitted ultrasound is commonly in the range of 1 MHz to 15 MHz. The ultrasound propagates through the material under investigation and reflects off of structures such as boundaries between adjacent tissue layers. As it travels, the ultrasonic energy may be scattered, resonated, attenuated, reflected, or transmitted. A portion of the reflected signals are returned to the transducers and detected as echoes. The detecting transducers convert the echo signals to electronic signals and furnish them to a beamformer. The beamformer calculates locations of echo sources along a line (beam), and typically includes simple filters. After beam-forming, an image scan converter uses the calculated positional information resulting from several beams, to generate two dimensional data that can be presented as an image. In prior art systems, the image formation rate (i.e., the frame rate) is limited by at least a pulse round trip time. The pulse round trip time is the time between the transmission of ultrasonic sound into the media of interest and the detection of the last reflected signals.
As an ultrasound pulse propagates through a material under investigation, additional harmonic frequency components are generated. These additional harmonic frequency components continue to propagate and, in turn, reflect off of, or interact with, other structures in the material under investigation. Both fundamental and harmonic signals are detected. The analysis of harmonic signals is generally associated with the visualization of boundaries or image contrast agents designed to re-radiate ultrasound at specific harmonic frequencies.
FIG. 1 shows a prior art ultrasound system, generally designated 100. The ultrasound system 100 includes an element array 105 of transducer elements 110A-110H, a backing material 120, and a matching layer 130. Backing material 120 is designed to support element array 105 and dampen any ultrasound energy that propagates toward backing material 120. Matching layer 130 transfers ultrasound energy from transducer elements 110A-110H into a material of interest (not shown). Transducer elements 110A-110H are each individually, electronically coupled by conductors 115 and 117, through a transmit/receive switch 140 to a beam transmitter 150. In the current art, transducer elements 110A-110H are typically piezoelectric crystals. Transmit/receive switch 140 typically includes a multiplexer 145, allowing the number of conductors 117 to be smaller than the number of conductors 115. In the transmit phase, beam transmitter 150 generates electronic pulses that are coupled through transmit/receive switch 140, and applied to transducer elements 110A-110H and converted to ultrasound pulses 160. Taken together, ultrasound pulses 160 form an ultrasound beam 170 that probes a material of interest. Ultrasound beam 170 is focused to improve the spatial resolution of the ultrasound analysis.
FIGS. 2A and 2B show a prior art focusing method in which element array 105 is a phased array used to focus ultrasound beam 170 by varying the timing of electronic pulses 210 applied to transducer elements 110A-110H. Electronic pulses 210, with different delay times, are generated at beam transmitter 150. When electronic pulses 210 are converted to ultrasound pulses 160 by transducer elements 110A-110H, they form ultrasound beam 170 directed at a focal point 230. FIGS. 2A and 2B show two series of electronic pulses 210 each with a different set of delay times resulting in different focal points 230. In a similar manner, phased excitation of array 105 is used to direct (steer) ultrasound beam 170 in specific directions.
Ultrasound system 100 sends a series of ultrasound beam 170 through different paths to form an image with a cross-sectional area greater than the width of each individual ultrasound beam 170. Multiple beams are directed from ultrasound system 100 in a scanning or steering process. An ultrasound scan includes transmission of more than one distinct ultrasound beam 170 in order to image an area larger than each individual ultrasound beam 170. Between each transmit phase a receive phase occurs during which echoes are detected. Since each ultrasound beam 170, included in the ultrasound scan, requires at least one transmit/receive cycle, the scanning processes can require many times the pulse round trip time. Optionally, an ultrasound beam 170 is transmitted in several transmit/receive cycles before another ultrasound beam 170 is generated. If ultrasound transducers 110A-110H move relative to the material under investigation during the scanning process undesirable artifacts can be generated.
FIG. 3A through 3E show a prior art scanning process in a transducer array 310 of eight transducer elements designated 110A through 110H. Electrical pulses are applied to subsets 320A-320E of the eight transducer elements 100A-110H. For example, FIG. 3A shows ultrasound beam 170A formed by subset 320A including transducer elements 110A-110D. The next step in the scanning process includes ultrasound beam 170B formed by subset 320B including transducer elements 110B-110E as shown in FIG. 3B. Subset 320B includes most (seventy-five percent) of the transducer elements 110A-110H found in subset 320A. Subset 320A and subset 320B differ by two transducer elements 110A-110H, the difference includes the inclusion of one and the removal of another. In the example shown, the center of ultrasound beam 170B passes through focal point 230 and is displaced from the center of ultrasound beam 170A by a distance equal to one transducer element 110. As illustrated by FIGS. 3C through 3E, the process continues, each subset 320C through 320E, used to produce each ultrasound beam 170C through 170E, is displaced by one transducer element 110 relative to the subset 320B through 320D used to generate the previous ultrasound beam 170B through 170D. Echoes detected in the receive phase that occurs between each ultrasound beam 170 transmission are used to generate beam echo data. Analyses of the beam echo data are combined and scan converted to form an image and the scan process is repeated to produce multiple images. The subsets 320A-320E of transducer elements 110A-110H used to produce ultrasound beams 170A-170E are selected using an array of switches and multiplexer 145 (FIG. 1). These switches are typically located in transmit/receive switch 140 (FIG. 1).
FIGS. 4A through 4E show prior art examples of the states of switches 410A-410H used to generate five consecutive ultrasound beams 170A-170E (FIG. 3). The state of each switch 410 determines which of transducer elements 110A-110H (FIG. 3) are coupled to beam transmitter 150 and therefore excited. For example in FIG. 4A, the first four switches 410A-410D are closed and the second four switches 410E-410H are open. This condition results in a beam 170A generated by excitation of the first four transducer elements 110A-110C as in FIG. 3A. In FIG. 4B, the first switch 410A is open, the next four switches 410B-410D are closed, and the last three switches 410E-410H are open. As illustrated in FIG. 3B, this change in switch 410 settings positions the center of the resulting ultrasound beam 170B a distance, approximately equal to the width of one transducer element 110, from the center of the previous ultrasound beam 170A. In FIG. 4C the first two switches 410A and 410B are open, the next four switches 410C-410F are closed, and the last two switches 410G and 410H are open. This switch 410 setting results in ultrasound beam 170C displaced by one transducer element 110 from ultrasound beam 170B, as illustrated in FIG. 3C. FIGS. 4D and 4E illustrate switch 410 settings used to produce ultrasound beams 170D and 170E shown in FIGS. 3D and 3E, respectively.
Some prior art systems use electronically controlled switches 410 and multiplexer 145 (FIG. 1) to select the subset 320 (FIG. 3) of transducer elements 110A-110H used to produce ultrasound beam 170. Regardless of the control means, the subsets 320 of transducer elements 110A-110H used to produce ultrasound beam 170, during the scanning process, differ by the inclusion and exclusion of one transducer element 110. The time required to scan over a large array of transducer element 110 is a significant factor in the time required to form an ultrasound image. Arrays optionally include a greater number of transducer element 110, for example, sixty-four, one hundred and twenty-eight, or more. When used to control arrays with greater numbers of transducer element 110, transmit/receive switch 140 includes multiplexer 145 that couples more than one beam transmitter 150 output to a greater number of transducer elements 110. Except at the edges of element transducer array 310, every output of beam transmitter 150 is coupled to every transducer element 110. This coupling is required since a transducer element 110 in the center of transducer array 310 is alternatively excited by all of the outputs of beam transmitter 150. For example, as illustrated in FIGS. 3A-3E, transducer element 110D is included in different positions within the four subsets 320A-320D. Each position is typically associated with a specific output of beam transmitter 150. In the prior art, a typical transducer element 110 is used to generate four, eight, or more distinct ultrasound beam 170.